It is sometimes desired to provide a tire with a rubber tread to promote reduced rolling resistance for the tire itself and thereby improved fuel economy for an associated vehicle, as well as reduced heat buildup in the tire tread during tire service to promote tire tread durability.
To promote one or more of such desirable properties, it is sometimes desired to promote a reduction in the hysteretic property of the tread rubber.
To promote a reduction in the hysteresis of the tread rubber (e.g. to promote a reduction in the rubber's physical rebound property), it may desired to significantly reduce its reinforcing carbon black filler content with an attendant increase in its precipitated silica filler content.
However, a significant reduction in rubber reinforcing carbon black content of the tread rubber, for example only up to 30 phr or even only up to 20 phr of rubber reinforcing carbon black, renders a significant reduction in its thermal and electrical conductivity particularly as the rubber reinforcing carbon black content falls below what is known as its percolation point. Such phenomenon is well known to those having skill in such art.
It would therefore be advantageous to provide a tread rubber composition with significantly reduced rubber reinforcing carbon black content but, nevertheless, to promote thermal conductivity and electrical conductivity for the tire tread rubber composition.
For such challenge, it is proposed to provide a very small content of graphene dispersion in the tread rubber composition to promote its thermal conductivity as well as its electrical conductivity.
However, adequate dispersion of the graphene filler in the tread rubber composition is desired and such dispersion may be challenging to achieve.
For such challenge, it is proposed to provide an inclusion of graphene filler in a tread rubber composition where the surface of the graphene is functionalized through non-damaging pi-pi (π-π) network interaction with a poly(phenylene ethynylene) oligomer containing a moiety promoting a pi-pi (π-π) bonding of the oligomer to the surface of the graphene to thereby functionalize the graphene and another different moiety of polysulfide for interaction with diene-based elastomers contained in the tread rubber composition. In this sense, then, the poly(phenylene ethynylene) oligomer may be considered as acting somewhat as a coupling agent to promote bonding of the graphene filler to the diene-based elastomer(s) in the tread rubber composition, and, further, to promote creation of a beneficial dispersion of the graphene filler in the tread rubber composition.
Historically, graphene has been suggested for inclusion in rubber compositions, including tire treads, for various purposes. For example, and not intended to be limiting, see Patent Publications: U.S. Pat. No. 6,476,154, US2006/0061011, US2010/0078194, US2011/0146859, WO2003/060002, DE 102007056689, JP2009/046547 and KR 100635604. It has also been proposed to functionalize carbon nanotubes for inclusion in a rubber composition by promoting pi-pi (π-π) bonding of compounds to their surface for use in rubber compositions. For example see U.S. Pat. No. 9,162,530.
In one embodiment, use of functionalized graphene in a diene-based elastomer containing rubber composition may be provided which contains particulate reinforcement in a form of combination of rubber reinforcing carbon black and precipitated silica (amorphous synthetic precipitated silica) together with a silica coupler for the precipitated silica.
In one embodiment, at least a portion of the precipitated silica may be provided as a pre-hydrophobated precipitated silica in a form of composite of precipitated silica pre-reacted with a silica coupling agent prior to its addition to the rubber composition.
The graphene is conventionally nano-sized particles in a sense of having an average thickness in a range of from about 1 nm to about 20 nm and an average L/D (lateral length to thickness dimension, or ratio) in a range of from about 10/1 to about 10,000/1.
Such graphene is conventionally prepared, for example, by a modified Hummers Method, in which natural graphite is treated with potassium permanganate and highly concentrated sulfuric acid. This reaction disrupts the delocalized electronic structure of graphite and imparts a variety of oxygen-based chemical functionalities to the surface, resulting in graphite oxide with a typical carbon-to-oxygen ratio of about 1.4 as measured by elemental analysis by combustion. The resulted graphite oxide is subsequently exfoliated either by a rapid thermal expansion at temperatures in the range of about 700° C. to 1200° C., or by chemical exfoliation followed by reduction through hydrazine.
As indicated, a significant aspect of this invention is to provide the graphene in a functionalized form to promote a thermally conductive property to the rubber composition and to rely on the precipitated silica, together with a coupling agent for the precipitated silica, without an appreciable rubber reinforcing carbon black content to provide reinforcement for the rubber composition. The functionalization of the graphene is envisioned as promoting its dispersion within the rubber composition and to promote rubber-to-filler (the graphene filler) bonding within the rubber composition to promote low rolling resistance and treadwear resistance for a tire tread rubber composition as well as to promote thermal conductivity for the rubber composition and thereby the tire tread, sidewall and other internal components such as a rubber chimney (radially extending rubber element) extending from the tread base rubber layer of a tire tread through an overlaying tread cap rubber layer to the outer tread cap rubber intended to be ground contacting.
It is believed that a tire with a component such as a tread which contains a minimal, if any, rubber reinforcing carbon black together while containing the aforesaid functionalized graphene content is a departure from past practice particularly when at least a portion of its precipitated silica filler reinforcement is a pre-hydrophobated precipitated silica (precipitated silica pre-hydrophobated prior to its addition to the rubber composition).
It is envisioned, in one embodiment, that the graphene may be coupled to the diene-based elastomer(s) of the rubber composition by a graphene coupling agent in the form of an oligomer to thereby rely on pi-pi (π-π) bonding of the graphene to the diene-based elastomer(s) and sulfur bonding to, or sulfur co-curing with, the diene based elastomer(s). There, the graphene coupling agent has a moiety with pi electron configuration to bond to the graphene and another different moiety comprised of a polysulfide based moiety to interact with carbon-to-carbon double bonds of the diene-based elastomer(s) of the rubber composition or comprised of a carbon-to-carbon double bond to sulfur co-cure with the diene-based elastomer(s).
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” and “uncured”, unless otherwise indicated.